A dual channel memory system has a memory controller with two independent channels. So for a data read that needs, say, 1000 memory accesses, a single channel controller has to perform 1000 reads, whereas with a dual channel controller, each channel only needs to do 500, giving much better performance. That doesn't mean it will be automatically twice as fast for any given system, but it gives a considerable improvement in potential data transfer rate.
It's recommended for dual channel systems to use DIMMs matching in ratings including size. So it could be best to use two 4GB DIMMS.
Some systems can use dual channel mode with differing size SIMMs by running the larger DIMM partly in dual channel mode, and the remainder that doesn't pair with the smaller SIMM in single channel mode. Not all systems can do this, you would need to find out if the specifications of your laptop indicate whether it can.
If it cannot, you would end up with the whole 12GB running in the slower single channel mode, so you'd probably be better off with the two 4GB DIMMs, depending on your RAM requirements in typical usage. If it can do split mode, you could have the 12GB with the first 8GB running in dual channel mode, and the last 4GB being that bit slower, but only being used if the system needs more than 8GB of memory active.
Edited by Platypus, 09 January 2017 - 04:43 AM.